1. Field of the Invention
This invention relates to a magnetic toner used in image forming processes such as electrophotography, electrostatic recording and magnetic recording, and also relates to an image forming method employing such a magnetic toner.
2. Related Background Art
A number of methods are hitherto known for electrophotography. In general, copies are obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and, by various means, subsequently developing the latent image by the use of a toner to form a toner image as a visible image, transferring the toner image to a transfer medium such as paper if necessary, and then fixing to the transfer medium the toner image formed thereon, by heating, pressing or heat-and-pressure means.
As methods by which the electrostatic latent image is formed into a visible image, developing methods such as cascade development, magnetic brush development and pressure development are known in the art. Another method is also known in which, using a magnetic toner and using a rotary sleeve internally provided with a magnet, the magnetic toner on the rotating sleeve is caused to fly to a photosensitive member under application of an electric field.
One-component development systems require no carrier such as glass beads, iron powder or magnetic ferrite particles required in two-component development systems, and hence can make developing assemblies themselves small-sized and light-weight. Meanwhile, since in the two-component development systems the concentration of toner in developer must be kept constant, a device for detecting toner concentration so as to supply the toner in the desired quantity is required, resulting in a more increase in size and weight of the developing assemblies. In the one-component development system, such a device is not required, and hence the developing assemblies can be made small and light-weight, as is preferable.
As printers, LED printers or LBP printers prevail in the current market. As a trend of techniques, there is a tendency toward higher resolution. Printers having a resolution of 240 or 300 dpi are being replaced by printers having a resolution of 400, 600 or 800 dpi. Accordingly, due to such a trend, the developing systems are now required to achieve a high minuteness. Copying machines have also made progress to have high functions, and hence the trend is toward digital systems. In this trend, the method chiefly employed is one in which electrostatic latent images are formed by using a laser. Hence, the copying machines also trend toward a high resolution and, as in the case of printers, it has been sought to provide a developing system with high resolution and high minuteness. Accordingly, toners are also being made to have smaller particle diameters, and toners having small particle diameters with specific particle size distributions are proposed in Japanese Patent Applications Laid-open No. 1-112253, No. 1-191156, No. 2-214156, No. 2-284158, No. 3-181952 and No. 4-162048.
In copying machines, the two-component developing system is most prevalent for medium-speed machines and high-speed machines. This is because, in the case of machines with a certain large size, stability in long-term use at high speed is more important than size or weight of the developing unit. Toners for two-component developers are commonly composed of a coloring component such as carbon black and other components almost held by polymers. Hence, toner particles are light and have no power to adhere to carrier particles, other than the electrostatic force, to tend to cause the scatter of toner especially in high-speed development, tend to cause contamination of lenses, original glass plates and transport assemblies in copying machines during long-term use, and tend to damage the stability of images. Accordingly, it is proposed to use a toner for two-component developers that is comprised of toner particles incorporated with a magnetic material so as to make the toner heavy and at the same time attractable to magnetic carrier particles by virtue not only of the electrostatic force but also the magnetic force so that the toner can be prevented from scattering.
Hence, magnetic toners containing magnetic materials have become increasingly important.
In the magnetic one-component developing system, development is carried out while the magnetic toner is formed into chains (commonly called "ears"), and hence the resolution of an image in the lateral direction tends to be poor compared with that in the longitudinal direction. For example, a phenomenon known as "smeared image trailing edge" tends to occur, which is due to the protrusion of ears into a non-image area of the latter half of a developed image, and also coarse images tend to occur compared with the two-component developing system. Accordingly, as a method for improving image reproducibility, it can be considered effective to make the ears of the magnetic toner shorter and denser. As a means therefor, it can be contemplated to decrease the proportion of the amount of a magnetic material in the magnetic toner, or to use a method in which a toner layer thickness control member is firmly brought into touch with the toner carrying member. However, an attempt to decrease the proportion of the amount of a magnetic material in the magnetic toner commonly results in an excessive increase in charge quantity of the magnetic toner to tend to cause the phenomenon of charge-up and cause a decrease in image density and an increase in fog, bringing about a lowering of image quality level.
The relationship between the intensity of magnetization of magnetic toners and the shape of each ear is understood as follows: When the intensity of magnetization of a magnetic toner is great, a strong attraction force in the direction of the magnetic field and a strong repulsion force in the direction perpendicular to the magnetic field act between magnetic toner particles. Hence, when the intensity of magnetization is great, the ears formed by the magnetic toner become long, the ears formed on the toner carrying member become loose and each ear becomes slender. Inversely, when the intensity of magnetization of a magnetic toner is small, the ears become short and the ears formed on the toner carrying member become dense, but each ear becomes thick and short because of no loosening of the combination between magnetic toner particles, resulting in an aggregated state. Hence, in the latter case, the magnetic toner particles present inside the ears have less opportunities to contact with the surface of the toner carrying member to tend to be insufficiently statically charged. Such magnetic toner particles insufficiently charged tend to cause fog on images, resulting in a lowering of image quality level.
In recent years, from the viewpoint of environmental protection, the primary charging process utilizing corona discharge and the transfer process utilizing corona discharge, which have been conventionally used, are making way for the primary charging process and/or transfer process making use of a contact member on the photosensitive member, which is/are becoming prevalent. For example, processes concerning contact charging or contact transfer are proposed in Japanese Patent Applications Laid-open No. 63-149669 and No. 2-123385. A conductive flexible charging roller is brought into contact with an electrostatic latent image bearing member and the electrostatic latent image bearing member is statically charged while applying a voltage to the conductive flexible charging roller, followed by exposure to form an electrostatic latent image. The electrostatic latent image is developed to form a toner image. Thereafter a conductive transfer roller to which a voltage has been applied is pressed against the electrostatic latent image bearing member, during which a transfer medium is passed between them, and the toner image held on the electrostatic latent image bearing member is transferred to the transfer medium, followed by the step of fixing to obtain a fixed image.
Since, however, in such a contact transfer system utilizing no corona discharge, the transfer means presses the transfer medium against the electrostatic latent image bearing member at the time of transfer, the toner image undergoes pressure when the toner image formed on the electrostatic latent image bearing member is transferred to the transfer medium, tending to cause a problem of partial faulty transfer, i.e., what is called "blank areas caused by poor transfer".
Moreover, in the contact transfer system, the electrical discharge produced between the charging roller and the electrostatic latent image bearing member more greatly physically and chemically acts on the surface of the electrostatic latent image bearing member than in the corona charging system. In particular, in the combination of an OPC photosensitive member with blade cleaning, problems such as melt adhesion of toner onto the OPC photosensitive member and faulty cleaning tends to occur, which are caused by a deterioration of the OPC photosensitive member surface. Combination of direct charging/organic photosensitive member/magnetic one-component developing system, contact transfer/blade cleaning can make image forming apparatus low-cost, small-sized and light-weight with ease, and is a system preferable for copying machines, printers and facsimile machines used in the field where the low cost, small size and light weight are demanded.
Accordingly, magnetic toners used in such an image forming method are required to have good releasability and lubricity. Incorporation of a silicone compound into a toner is proposed in Japanese Patent Publication No. 57-13868, Japanese Patent Applications Laid-open No. 54-58245, No. 59-197048, No. 2-3073 and No. 3-63660 and U.S. Pat. No. 4,517,272. Since, however, in such a method the silicone compound is directly added in toner particles, the silicone compound, having no compatibility with binder resins, has so poor a dispersibility in toner particles that the charging performance of the toner particles tends to be non-uniform to cause the problem of a lowering of developing performance in long-term repeated use.
In recent years, from the viewpoint of environment protection, reclaimed paper has come to be used as copy paper. Since, however, the reclaimed paper may produce paper dust and filler powder in a large quantity when used, the problems of faulty cleaning and melt adhesion of toner tend to occur. These problems must be overcome in order to accomplish image forming apparatus that are small-sized, light-weight and low-cost and which can obtain images with a high resolution and a high minuteness while taking into account environmental problems.